1. Technical Field
The present invention relates generally to electric wave transmission systems wherein electromagnetic wave energy is guided or constrained, and more particularly to mode converters for changing guided waves having one field configuration to a different field configuration, wherein the original and the changed waves each have a longitudinal electric or magnetic field component.
2. Background Art
Many radio frequency applications today require electromagnetic energy at high power levels and at frequencies in the 1 to 150 GHz range. Some common examples are radio frequency heating, radar, satellite communications, and high energy physics.
Waveguides are used to propagate electromagnetic energy within much of the equipment used by such applications. A waveguide is usually categorized by its shape and its mode of operation. Waveguide shape is simply the predominant cross-sectional shape, and is most often simply spoken of as being “rectangular” or “circular.” This coincidentally defines a “waveguide axis” that is perpendicular to and centered through the waveguide cross-section.
Waveguide modes are categorized according to the nature of the longitudinal components of the electric (EZ) and magnetic (HZ) fields of the electromagnetic energy that they are used with, i.e., with respect to field vectors perpendicular to the waveguide axis. Such modes are generally referred to as being either “transverse-electric” (TE), meaning that the electric field vector is perpendicular to the waveguide axis or “transverse-magnetic” (TM), meaning that the magnetic field vector is perpendicular to the waveguide axis. The modes are further categorized by subscripts mathematically derived from EZ and HZ. Numerous texts describe the derivation of such subscripts, but that process is not relevant here.
FIGS. 1a-1c (background art) depict some conventional waveguide examples and particular aspects of them that serve to illustrate various important points. FIG. 1a shows a rectangular waveguide operating in TE1,0 mode. In a rectangular waveguide, TE1,0 mode is dominant. FIG. 1b shows a circular waveguide operating in TE1,1 mode. In a circular waveguide, TE1,1 mode is dominant. Other modes are possible and useful, however, and FIG. 1c depicts one of particular interest. FIG. 1c shows a circular waveguide in TM0,1 mode.
In FIGS. 1a, 1b solid arrowed lines depict the electric field (E) and in FIG. 1c dashed arrowed lines depict the magnetic field (H). By comparison of FIG. 1b and FIG. 1c it can now be seen why TM0,1 mode is also termed a “circularly symmetric” mode.
Designing waveguides that efficiently propagate electromagnetic energy in one direction and in one mode of operation is generally a mature art. Unfortunately, many important applications today require more, changing from one waveguide shape to another, changing from one waveguide mode of operation to another, or changing the direction of energy propagation. In some critical applications, such as scanning radars and satellite communications, all of these are needed.
When changing the direction of propagation a small amount of rotation can usually be accommodated by using flexible coaxial cables or waveguides. This approach has been used in radars for more than 50 years. This does not, however, provide for continuous 360-degree rotation.
When substantial or full rotational capability in an electromagnetic wave transmission path is desirable or necessary, the rotary joint is the preferred apparatus. In general, a rotary joint desirably operates over the full rotation range with minimum insertion loss and voltage standing wave ratio (VSWR), minimum distortion of the electromagnetic wave, and with minimum variation over the frequency band as rotation takes place.
FIG. 2 (prior art) is a cross-sectional view of a rotary waveguide joint 1 in accord with the teachings of U.S. Pat. No. 2,708,263 by Walters. This example has two major sections 2 that are rotatably joined by a rotation mechanism 3. Each major section 2 includes a rectangular waveguide sub-section 4 and a circular waveguide sub-section 5. The rectangular waveguide sub-sections 4 each have a waveguide axis 6 and the circular waveguide sub-section 5 share a common waveguide axis 7. To facilitate understanding the rotary waveguide joint 1 is shown with the axes 6, 7 all co-planar. Of course, this is not always the case in actual operation.
In use, the rotary waveguide joint 1 accepts electromagnetic energy in TE1,0 mode through one rectangular waveguide sub-section 4, converts it to the circularly symmetric TM0,1 mode and propagates it through the corresponding circular waveguide sub-section 5. The rotation mechanism 3 includes a break between the circular waveguide sub-sections 5 that acts as a small-gap radio frequency choke to provide an effective short-circuit at the frequency of the electromagnetic energy. This permits the electromagnetic energy to be propagated into and through the other circular waveguide sub-section 5, and then converted back to TE1,0 mode and propagated through the remaining rectangular waveguide sub-section 4.
Efficient propagation particularly needs to occur regardless of the rotational orientations of the two major sections 2, and that is why the electromagnetic energy is preferably in circularly symmetric TM0,1 mode as it passes through the two circular waveguide sub-sections 5. In this mode the orientation of the electric (E) and magnetic (H) field patterns is independent of the rotational relationship of the two major sections 2 of the rotary waveguide joint 1.
With reference again briefly to FIG. 1c, it can be seen that TM0,1 mode is characterized by a radially extending electric field with constant amplitude and phase as a function of angular rotation about the periphery. This characteristic particularly makes this mode suitable for use with rotary joints. This also makes this mode suitable for use in applications where structural rotation is not necessarily employed, such as in particle accelerators in modern physics laboratories.
One means of exciting the TM0,1 mode is with a step transition at an interface where a rectangular waveguide forms a right angle junction to a circular waveguide. This suppresses the otherwise dominant TE1,1 mode. In the example in FIG. 2, each of the major sections 2 has such a step transitions 8 where the rectangular waveguide sub-sections 4 transition into their respective circular waveguide sub-sections 5.
FIGS. 3a, 3b (background art) depicts a simplified waveguide structure 10 having a step transition 11. FIG. 3a shows the waveguide structure 10 in top plan view and FIG. 3b shows the waveguide structure 10 in side cross-section view. An important point to be noted here is that the waveguide structure 10 is a very difficult one to manufacture. The same is true of the elements of the rotary waveguide joint 1 in FIG. 2.
As is well known in the art, when using devices for transferring high power electromagnetic waves it is necessary that sharp edges be blended (rounded or smoothed), and to generally have as few structural changes and connections as possible. This prevents arcing and contributes to more efficient energy propagation. Accomplishing this is difficult and expensive in device manufacture, however, when edges are not complete circles or even straight edges, and particularly when an edge is not accessible for finishing. It follows that the example waveguide structures 1, 10 shown, especially at the step transitions 8, 11, require design compromises or the use of very extra-ordinary machining techniques.
It follows that what is need is an improved mode transducer structure.